High speed linear variable gain amplifier architecture

ABSTRACT

A two stage amplifier circuit ( 10 ), the first stage ( 12 ) comprising a modified quad configuration and the second stage ( 14 ) comprising a translinear current amplifier configuration. The present invention achieves the advantages of fast response time, low distortion and improved bandwidth. The current gain of the second stage is represented by: 
     ( IA   out1   −IA   out2 )/( I   out1   −I   out2 )=(1+ R   123   /R   124 )·( I   135   /I   134 )·( A /(1+ A )) 
     where A=g mQ109 ·R 124 .

FIELD OF THE INVENTION

[0001] This invention relates to amplifiers and in particular, amplifiers having variable gain, large bandwidth and low distortion.

BACKGROUND OF THE INVENTION

[0002] Amplifiers are used to manipulate various signals within a circuit. The topology of the amplifier affects various operating aspects of the operating amplifier. For example, some amplifiers can deliver a high output current to a load. Other amplifiers can produce an output voltage swing that is approximately equal to the magnitude of the power supply of the amplifier circuit. Some amplifiers must provide an output with low cross-over distortion whereas other amplifiers are required to maintain gain and stability at high frequencies. These different requirements place constraints upon the design of the amplifier. It is often desirable in an amplifier circuit to have variable gain, large bandwidth and low distortion. Conventional solutions use attenuators as front ends followed by high gain, closed-loop amplifiers or multiple lower gain closed-loop amplifiers. Disadvantageously, these conventional solutions require much higher FT (factor of ten) amplification to achieve these results.

SUMMARY OF THE INVENTION

[0003] The present invention achieves technical advantages as a variable gain amplifier with wide bandwidth and low distortion by using two stages, a quad input stage with emitter degeneration and translinear current amplifier second stage.

[0004] The first stage quad configuration allows a constant DC output level. The output current of the quad is then fed into a resistance shunt current feedback amplifier with Darlington/level shift input stage to reduce transistor beta loading effects as well as allowing the largest dynamics out of the stage when a current to voltage and common mode feedback circuit are implemented in the same stage.

[0005] The second stage presents a low input impedance to the quad allowing optimization of the quad with minimize loss of bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a schematic circuit diagram of the present invention;

[0007]FIG. 2 is a schematic of a conventional npn current feedback topology;

[0008]FIG. 3 is a a simplified equivalent circuit shown in FIG. 2; and

[0009]FIG. 4 is a schematic circuit diagram of the translinear loop (Q₁₀₉ to Q₁₁₂) of the second stage of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0010] The amplifier of the present invention, shown at 10 in FIG. 1, is a combination of a quad 12 current drive circuit and a translinear current amplifier 14. The quad 12 provides a constant DC output level. As used herein, quad refers to a series-parallel configuration of four transistors. The quad is modified with emitter degeneration. The output drive current of the quad 12 is fed into the translinear current amplifier 14 comprising a resistive shunt current feedback amplifier with Darlington/level shift input stage. This arrangement reduces transistor beta loading effects as well as allowing the largest dynamics out of the stage when a current to voltage and common mode feedback circuit are implemented in the same stage. Translinear loop transistors 109 (“Q₁₀₉”), 110 (“Q₁₁₀”), 111 (“Q₁₁₁”) and 112 (“Q₁₁₂”) ensure a fast conveyence of the collector currents of Q₁₀₉ (“Q_(C109)”) and Q₁₁₂ (“Q_(C112)”) to the output. The current gain of the second stage is given as follows:

(IA _(out1) −IA _(out2))/(I _(out1) −I _(out2))=(1+R ₁₂₃ /R ₁₂₄)·(I ₁₃₅ /I ₁₃₄)·(A/(1+A))

[0011] where A=g_(mQ109)·R₁₂₄

[0012] The second stage 14 presents a low input impedance to the quad 12 allowing optimization of the quad 12 with minimum loss of bandwidth. Current to voltage conversion and common mode feedback implemented in the second stage 12 allows least delay, best distortion and highest bandwidth with such an architecture. The inherent all npn core variable gain amplifier 10 ensures the best possible bandwidth and flexibility of use on both all npn or complementary bipolar processes.

[0013] The equations below better illustrate the operation of the second stage.

[0014]FIG. 2 depicts a schematic of a conventional npn current feedback topology. From FIG. 3, we can derive the transfer function of FIG. 2. FIG. 3 is the equivalent circuit of FIG. 2. $\frac{I_{Q109}}{I_{in}}$ ${{1.\quad \frac{V_{i\quad n} - V_{out}}{R_{123}}} = {{I_{i\quad n}->V_{i\quad n}} = {V_{out} + {I_{i\quad n} \cdot R_{123}}}}};$ ${{{2.\quad \frac{V_{out}}{R_{124}}} + I_{Q109} + \frac{V_{out} - V_{i\quad n}}{R_{123}}} = 0};$   I_(Q109) = g_(mQ109) ⋅ V_(i  n) ${3.\quad V_{i\quad n}} = {\frac{I_{Q109}}{g_{mQ109}}.}$

[0015] Substituting equation (3) for equations (2) and (1) results in: ${4.\quad \frac{I_{Q109}}{g_{mQ109}}} = {{{V_{out} + {I_{i\quad n}R_{123}}}->V_{out}} = {\frac{- I_{Q109}}{g_{mQ109}} - {I_{i\quad n} \cdot R_{123}}}}$ ${{5.\quad \frac{V_{out}}{R_{124}}} + I_{Q109} + \frac{V_{out} - \frac{I_{Q109}}{g_{mQ109}}}{R_{123}}} = 0$

[0016] Substituting equation (4) for (5) and solving for $\frac{I_{Q109}}{I_{{i\quad n}\quad}}$

[0017] provides as follows: $\begin{matrix} {\frac{I_{Q109}}{I_{i\quad n}} = \quad \frac{g_{mQ109}\left( {R_{123} + R_{124}} \right)}{1 + {g_{mQ109} \cdot R_{124}}}} \\ {= \quad {\frac{\left( {g_{mQ109} \cdot R_{124}} \right)}{1 + {g_{mQ109} \cdot R_{124}}}\frac{\left( {R_{123} + R_{124}} \right)}{R_{124}}}} \\ {\frac{I_{Q109}}{I_{i\quad n}} = \quad {{\frac{A}{1 + A}\left( {1 + \frac{R_{123}}{R_{124}}} \right)\quad {where}\quad A} = {g_{mQ109} \cdot R_{124}}}} \end{matrix}$

[0018] Referencing FIG. 4, we can derive the transfer function for the translinear loop: 6.  V_(be108) − V_(be112) + V_(be111) − V_(be110) = 0 $\quad {{{V_{Tin}\left( \frac{I_{Q109}}{I_{s}} \right)} - {V_{Tin}\left( \frac{I_{Q112}}{I_{s}} \right)} + {V_{Tin}\left( \frac{I_{Q111}}{I_{s}} \right)} - {V_{Tin}\left( \frac{I_{Q110}}{I_{s}} \right)}} = 0}$ $\quad {{{\frac{I_{Q109}}{I_{Q112}} \cdot \frac{I_{Q111}}{I_{Q110}}}->{I_{Q109} \cdot I_{Q111}}} = {I_{Q110} \cdot I_{Q112}}}$

 I _(Q109) +I _(Q112) =I ₁₃₄ →I _(Q112) =I ₁₃₄ −I _(Q109)  7.

I _(Q110) +I _(Q111) =I ₁₃₅ →I _(Q111) =I ₁₃₅ −I _(Q110)  8.

[0019] Substituting equation (7) and (8) for (6):

I _(Q109)(I ₁₃₅ −I _(Q110))=I _(Q110)(I ₁₃₄ −I _(Q109))

(I _(Q109) ·I ₁₃₅)−(I _(Q109) ·I _(Q110))=(I _(Q110) I ₁₃₄)−(I _(Q109) ·I _(Q110))

I _(Q109) ·I ₁₃₅ =I _(Q110) ·I ₁₃₄ $\frac{I_{Q110}}{I_{Q109}} = \frac{I_{135}}{I_{134}}$

[0020] By combining the derivation of the transfer functions of FIGS. 3 and 4, we obtain: $I_{Q110} = {{I_{out}->\frac{I_{out}}{I_{Q109}}} = \frac{I_{135}}{I_{Q134}}}$ ${{{Then}\quad \frac{I_{out}}{I_{i\quad n}}} = {\frac{I_{out}}{I_{Q109}} \cdot \frac{I_{Q109}}{I_{i\quad n}}}};$ ${{\frac{I_{out}}{I_{Q109}} \cdot \frac{I_{Q109}}{I_{i\quad n}}} = {{\frac{I_{135}}{I_{134}} \cdot \frac{T}{1 + T}}\left( {1 + \frac{R_{123}}{R_{124}}} \right)}};$ ${{{Therefore}:\quad \frac{I_{out}}{I_{i\quad n}}} = {\left( {1 + \frac{R_{123}}{R_{124}}} \right)\left( \frac{R_{135}}{R_{134}} \right)\left( \frac{A}{1 + A} \right)}}\quad$ where  A = g_(m) ⋅ I_(Q109) ⋅ R₁₂₄ 

What is claimed is:
 1. An amplifier circuit, comprising: a first stage and a second stage, the first stage comprising a quad configuration and the second stage comprising a translinear current amplifier configuration; and a coupling circuit operably coupling the first stage and the second stage to reduce transistor beta loading effects.
 2. The amplifier circuit recited in claim 1, wherein the first stage quad configuration is modified using emitter degeneration.
 3. The amplifier circuit recited in claim 1, wherein the current gain of the second stage is given by: (IA _(out1) −IA _(out2))/(I _(out1) −I _(out2))=(1+R ₁₂₃ /R ₁₂₄)·(I ₁₃₅ /I ₁₃₄)·(A/(1+A))where A=g_(m Q109)·R₁₂₄; IA_(out1) is the amplified output collector current from Q₁₁₀; IA_(out2) is the output collector current from transistor Q₁₁₁; I_(out1) is the output current from Q₁₀₃ and Q₁₀₅ from the first stage quad, and I_(out2) is the collector current from Q₁₀₄ and Q₁₀₆ from the first stage quad; R₁₂₃ is the resistance value of the third resistor and R₁₂₄ is the resistance value of the fourth resistor; I₁₃₅ is the current through the fifth current source; and I₁₃₄ is the value of the current through the fourth current source.
 4. The amplifier circuit recited in claim 1, further comprising current to voltage conversion and common mode feedback in the second stage operable to provide high speed, low distortion and extended bandwidth.
 5. The amplifier circuit recited in claim 1, wherein the amplifier is formed of complementary bipolar devices.
 6. The amplifier circuit recited in claim 1 being adapted for use in an integrated circuit.
 7. The amplifier circuit recited in claim 1 being adapted for use in a variable gain amplifier.
 8. An amplifier circuit, comprising a first stage and a second stage, the first stage comprising a quad configuration, and the second stage comprising a resistive shunt current feedback amplifier with a Darlington/level shift input stage operable to reduce transistor beta loading effects.
 9. The amplifier circuit recited in claim 8, wherein the first stage quad is modified using emitter degeneration.
 10. An amplifier circuit, comprising: a first stage quad operable to bias transistors; and a coupling circuit coupling the first stage quad to a second stage translinear current amplifier operable to reduce transistor beta loading effects and provide improved slew rate, low distortion and extended bandwidth.
 11. The amplifier circuit recited in claim 10, wherein the first stage quad is modified using emitter degeneration.
 12. An amplifier circuit, comprising: a first stage comprising six transistors, two resistors and a current source configured as a quad; a second stage comprising eight transistors, four resistors, and six current sources configured as a translinear current amplifier; the first stage being coupled to the second stage operable to provide current gain of: (IA _(out1) −IA _(out2))/(I _(out1) −I _(out2))=(1+R ₁₂₃ /R ₁₂₄)·(I ₁₃₅ /I ₁₃₄)·(A/(1+A)) where A=g_(mQ109)·R₁₂₄; IA_(out1) is the amplified output collector current from Q₁₁₀; IA_(out2) is the output collector current from transistor Q₁₁₁; I_(out1) is the output current from Q₁₀₃ and Q₁₀₅ from the first quad stage quad, and I_(out2) is the collector current from Q₁₀₄ and Q₁₀₆ from the first quad stage; R₁₂₃ is the resistance value of the third resistor and R₁₂₄ is the resistance value of the fourth resistor; I₁₃₅ is the current through the fifth current source; and I₁₃₄ is the value of the current through the fourth current source.
 13. The amplifier circuit recited in claim 12 wherein the first stage quad is modified using emitter degeneration.
 14. The amplifier circuit recited in claim 12, further comprising: the first stage comprising two input voltage nodes, a first input voltage node being coupled to the base of a first transistor, a second input voltage node being coupled to the base of a second transistor; the emitter of the first transistor being coupled to the first terminal of a first resistor, the second terminal of the first resistor being coupled to the first node of a first current source and the second node of the first current source being coupled to a ground reference; the emitter of the second transistor being coupled to the first terminal of a second resistor, the second terminal of the second resistor being coupled to the first node of the first current source; the collector of the first transistor being coupled to the emitter of a third transistor, the emitter of a fourth transistor being coupled to the collector of the first transistor, the collector of the third transistor being coupled to a first current output node of the first stage; the collector of the fourth transistor being coupled to a second output current node of the first stage; the base of the third transistor being coupled to the base of a sixth transistor; the base of the fourth transistor being coupled to the base of a fifth transistor; the node coupling the base of the third transistor and the base of the sixth transistor having a first input control node; the node coupling the base of the fourth transistor and the base of the fifth transistor being coupled to a second control input node; the collector of the second transistor being coupled to the emitter of the fifth transistor and the emitter of the sixth transistor, the collector of the fifth transistor being coupled to the first output current node of the first stage, the collector of the sixth transistor being coupled to a second output current node of the first stage; the second stage comprising the base of a seventh transistor being coupled to the second output current node of the first stage, the emitter of the seventh transistor being coupled to a first node of the second current source and the base of an eighth transistor, the second node of the second current source being coupled to the ground reference; the collector of the seventh transistor being coupled to a voltage supply rail and the collector of the eighth transistor being coupled to the voltage supply rail; the second output current node from the first stage being coupled to a first terminal of a third resistor, the second terminal of the third resistor being coupled to the collector of a ninth transistor, the base of the ninth transistor being coupled to the emitter of the eighth transistor, the base of the ninth transistor also being coupled to a first terminal of a third current source; the second terminal of the third current source being coupled to the ground reference, the first terminal of the third current source also being coupled to the base of a tenth transistor; the emitter of the ninth transistor being coupled to a first terminal of a fourth current source, a second terminal of the fourth current source being coupled to the ground reference; the collector of the ninth transistor also being coupled to a first terminal of a fourth resistor, a second terminal of the fourth resistor being coupled to the voltage supply rail; the emitter of the tenth transistor being coupled to a first terminal of a fifth current source, a second terminal of the fifth current source being connected to the ground reference, the emitter of the ninth transistor also being coupled to the emitter of a twelfth transistor; the emitter of the tenth transistor also being coupled to the emitter of an eleventh transistor, the base of the eleventh transistor being coupled to a first terminal of a sixth current source, a second terminal of the sixth current source being coupled to the ground reference; the base of the twelfth transistor also being coupled to the first terminal of the sixth current source; the emitter of a thirteenth transistor being coupled to the first terminal of the sixth current source; the collector of the twelfth transistor being coupled to a first terminal of a fifth resistor, a second terminal of the fifth resistor being coupled to the voltage supply rail; the collector of the thirteenth transistor being coupled to the voltage supply rail, the base of a fourteenth transistor being coupled to the first output current node of the first stage, the base of the fourteenth transistor also being coupled to a first terminal of a sixth resistor, a second terminal of the sixth resistor being coupled to the collector of the twelfth transistor; the collector of the fourteenth transistor being coupled to the voltage supply rail, the emitter of the fourteenth transistor being coupled to a first terminal of a seventh current source, the second terminal of the seventh current source being coupled to the ground reference; the emitter of the fourteenth transistor also being coupled to the base of the thirteenth transistor; the collector of the tenth transistor being coupled to a node operable as an output current node of the second stage; and the collector of the eleventh transistor being coupled to a node operable as an output current node of the second stage.
 15. The amplifier circuit recited in claim 14, wherein the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth and fourteenth transistors comprise npn transistors.
 16. The amplifier circuit recited in claim 14 being adapted for use in an integrated circuit.
 17. The amplifier circuit recited in claim 14 being adapted for use as a variable gain amplifier. 